Microstrip structure including a signal line with a plurality of slit holes

ABSTRACT

A semiconductor device comprising a signal transmission line of a microstrip structure, capable of increasing the characteristic impedance of the signal transmission line and reducing coupling between a plurality of signal lines. In a signal transmission line of a microstrip structure composed of a signal line and a ground plate, the capacitance between wires is reduced and the characteristic impedance can be increased by forming holes in the signal line or in the ground plate. The coupling between a plurality of signal lines can also be reduced.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. application Ser. No.11/687,921, filed Mar. 19, 2007, which is a Divisional of U.S.application Ser. No. 09/664,094, filed Sep. 19, 2000, which claimspriority of Japanese Patent Application No. 11-266203, filed on Sep. 20,1999. The entire disclosures of the prior applications are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor integrated circuit,provided with an improved characteristic impedance of the signaltransmission line of a semiconductor integrated circuit, capable ofreducing coupling with other transmission lines.

2. Background Art

Conventionally, when providing a signal transmission line with amicrostrip structure constituted by a signal line and a ground plate ina semiconductor integrated circuit, two separate wiring layers for thesignal line and for the ground plate are used as shown in FIG. 8.

In order to increase the characteristic impedance of this signaltransmission line in a signal transmission line of a microstripstructure composed of the integrated signal lines and a ground plate,two methods have been adopted, one method is to make the signal linewidth narrower, and the other method is to increase the distance dbetween the signal line and the ground plate.

Furthermore, the other conventional technique is disclosed in JapaneseUnexamined Patent Application, No. Hei 5-343564, which determines thecharacteristic impedance of the transmission line by a mesh-like meshground plane, and the spaces corresponding to the pores of the mesh arecompensated for by the ground plate.

However, when the above-described first method for making the signalline width narrower is adopted, the problem arises that the maximuminput capacitance of a circuit connected to the end of the signaltransmission line becomes small.

When the above-described second method of increasing the distance dbetween the signal line and the ground plane is adopted, the problem isencountered that the thickness or the number of wiring layers increases,because it is necessary to increase the thickness of the interlayerfilms of the wiring layers or to increase the number of wiring layers.

When the third method disclosed in Japanese Unexamined PatentApplication, First Publication No. Hei 5-343564 is adopted, although itis possible to increase the impedance of the signal transmission line,the problem arises that, when assembling into an integrated circuit,since the signal lines cannot be formed under the ground plane, it isnot possible to avoid generating coupling between a plurality of signallines.

When coupling occurs, a deficiency arises that in the characteristicimpedance of a certain signal line will experience a dynamic change,because the capacitance between one signal line and its counter linechanges when the voltage level of the counter signal line changes.

In addition, the problem also arises in the case of using a mesh groundplane that it is difficult to evaluate the characteristic impedance,since the electro-magnetic field generated through small holes or spacescannot be accurately understood.

SUMMARY OF THE INVENTION

This invention has been made to solve the above-described problems. Itis, therefore, an objective of the present invention to provide asemiconductor device, capable of reducing the coupling between aplurality of signal lines by increasing the characteristic impedance ofthe signal transmission line of a microstrip structure, which can berealized in a semiconductor device.

According to the first aspect of the present invention, a semiconductorintegrated circuit comprises a signal transmission line of a microstripstructure composed of a signal line and a ground plate, wherein at leastone hole is formed in said signal line.*

According to the second aspect, in a semiconductor integrated circuitcomprising a signal transmission line of a microstrip structure composedof a signal line and a ground plate, at least one hole is formed in saidground plate.

According to the third aspect, in a semiconductor integrated circuitcomprising a signal transmission line of a microstrip structure composedof a signal line and a ground plate according to the second aspect, thesize of said at least one hole formed in said ground plate is determinedsuch that the AC coupling between the signal line and another signalline disposed close to one signal line and on the opposite side of saidground plate is decreased and the characteristic impedance of saidsignal transmission line is increased.

According to the fourth aspect, in a semiconductor integrated circuitcomprising a signal transmission line of a microstrip structure composedof a signal line and a ground plate according to the second aspect, thenumber of said at least one hole formed in said ground plate isdetermined such that the AC coupling between one signal line and anothersignal line disposed close to one signal line and on the opposite sideof said ground plate is decreased and the characteristic impedance ofsaid signal transmission line is increased.

According to the fifth aspect, in a semiconductor integrated circuitcomprising a signal transmission line of a microstrip structure composedof a signal line and a ground plate, at least one hole is formed in bothof said signal line and said ground plate.

According to the sixth aspect, in a semiconductor integrated circuitcomprising a signal transmission line of a microstrip structure composedof a signal line and a ground plate according to the fifth aspect, thesize of said at least one hole formed in said ground plate among thesignal line and the ground plate is determined such that the AC couplingbetween one signal line and another signal line disposed close to onesignal line and on the opposite side of said ground plate is decreasedand the characteristic impedance of said signal transmission line isincreased.

According to the seventh aspect, in a semiconductor integrated circuitcomprising a signal transmission line of a microstrip structure composedof a signal line and a ground plate according to the fifth aspect, thenumber of said at least one hole formed in said ground plate among holesformed in both of the signal line and the ground plate is determinedsuch that the AC coupling between one signal line and another signalline disposed close to one signal line and on the opposite side of saidground plate is decreased and the characteristic impedance of saidsignal transmission line is increased.

According to the eighth aspect, in a semiconductor integrated circuitcomprising a signal transmission line of a microstrip structure composedof a signal line and a ground plate according to the second aspect, saidat least one hole in said ground plate is formed at a position where theother signal line is not disposed or said at least one hole in saidground plate is made small so as to reduce the AC coupling with onesignal line when formed at a position where the other signal line isdisposed.

According to the ninth aspect, in a semiconductor integrated circuitcomprising a signal transmission line of a microstrip structure composedof a signal line and a ground plate according to the first aspect,where, instead of at least one hole formed in said signal line or insaid ground plate, a plurality of slit holes are formed by forming saidsignal line or said ground plate of a plurality of thin strips and byconnecting these thin strips at those terminal ends.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the structure of the signal transmissionline according to the first embodiment of the present invention.

FIG. 2 is a diagram showing the structure of the signal transmissionline according to the second embodiment of the present invention.

FIG. 3A and FIG. 3B are diagrams showing the relationship between signallines and a ground plate provided with a single slit.

FIG. 4 is a diagram showing the relationship between signal lines and aground plate provided with the slit divided into three slits.

FIG. 5 is a graph showing the relationship between the slit width andmagnitude of the coupling.

FIG. 6 is a graph showing the slit width in the ground plate and thecharacteristic impedance.

FIG. 7 is a diagram showing the structure of a signal transmission lineaccording to the third embodiment of the present invention.

FIG. 8 is a diagram showing the structure of a conventional signaltransmission line.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present embodiments of the present invention will bedescribed with reference to the drawings.

<First Embodiment>

FIG. 1 is a diagram showing the structure of the signal transmissionline according to the first embodiment of the present invention.

As shown in FIG. 1, in the signal transmission line of the microstripstructure realized in a semiconductor device according to the presentembodiment, at least one hole 13 (normally, a plurality of holes 13) isformed on a signal line of a microstrip structure composed of a signalline 11 and a ground plate 12. The hole 13 is shown to have an aperturesize which is smaller than a width of the signal line 11.

In a signal transmission line of the microstrip structure, thecharacteristic impedance Z is expressed by the equation,Z=(L/C)^(1/2)where C represents the inter-wiring capacitance per unit length, and Lrepresents the inductance of the wiring.

The provision of a hole (or a plurality of holes) in the signal line 11lengthen the electric force line generated between the signal line 11and the ground plate 12 and the distance between the signal line 11 andthe ground plate 12 is apparently elongated equivalently, which resultsin a decrease of the inter-wiring capacitance C.

When the velocity of the electro-magnetic wave is represented by v, thevelocity v is constant and is expressed by the following equation.v=1/(LC)1^(1/2)

Thus, when the capacitance C decreases, the wiring inductance Lincreases.

Consequently, the characteristic impedance can be increased by formingmore than one hole in the signal line.

In addition, the increase of the characteristic impedance as shown abovemakes it possible to suppress the reduction of the maximum inputcapacitance of the circuit connected at the end of the signaltransmission line, and the increase of the characteristic impedance doesnot require an increase in the number of wiring layers.

The size of the hole 13 in the signal line shown in FIG. 1 can bedetermined as follows. The size of the hole is determined so as toreduce the wiring resistance as much as possible. At the same time, thesize of the hole is determined so as to sufficiently increase thecharacteristic impedance between the signal line 11 and the ground plate12 as much as possible.

The number of holes 13 in the signal line 11 may be determined so as tofulfill the following conditions. The number can be decreased to reducethe wiring resistance as much as possible, and the number can beincreased to increase the characteristic impedance as much as possible.

It is noted that, instead of forming one signal line as shown in FIG. 1,the signal line may be formed by a plurality of signal line stripsspaced apart from each other by slits and the plurality of signal linestrips are connected at a certain portion such as both termination endsfor forming a plurality of slit-like holes.

<Second Embodiment>

FIG. 2 is a diagram showing the structure of the signal transmissionline according to the second embodiment of the present invention.

As shown in FIG. 2, in the signal transmission line of the microstripstructure realized in a semiconductor device according to the secondembodiment, at least one hole 24 (normally, a plurality of holes 24) isformed on the ground plate 22 of a microstrip structure composed of asignal line 21 and a ground plate 22.

In the signal transmission line of the microstrip structure, thecharacteristic impedance Z is expressed by the equation,Z=(L/C)^(1/2)where C represents the inter-wiring capacitance per unit length, and Lrepresents the inductance of the wiring.

The provision of a hole (or a plurality of holes) in the ground plate 22lengthen the electric force line generated between the signal line 11and the ground plate 12 and the distance between the signal line 11 andthe ground plate 12 is apparently elongated equivalently, which resultsin a decrease of the inter-wiring capacitance C.

When the velocity of an electro-magnetic wave is represented by v, thevelocity v is constant and is expressed by the following equation.v=1/(LC)^(1/2)

Thus, when the capacitance C decreases, the wiring inductance Lincreases.

Consequently, the characteristic impedance can be increased by formingmore than one hole in the ground plate 22.

As described above, it becomes possible to suppress the reduction of themaximum input capacitance of the circuit connected at the end of thesignal transmission line, and it is not necessary to increase the numberof wiring layers.

The size of the hole 24 in the ground plate shown in FIG. 2 can bedetermined as follows. The size of the hole is determined to reduce theAC coupling between two signal lines 21 adjacent to each other above thehole 24 in the ground plate 22 as much as possible. At the same time,the size of the hole is determined to sufficiently increase thecharacteristic impedance between the signal line 11 and the ground plate12 as much as possible.

The number of holes 24 in the ground plate shown in FIG. 2 can bedetermined so as to fulfill the following conditions. The number can bedecreased to reduce the AC coupling between two signal lines 21 adjacentto each other above hole 24 in the ground plate 22 as much as possible,and the number of the holes is determined to increase the characteristicimpedance between the signal line 21 and the ground plate 22 as much aspossible.

Next, the magnitude of the coupling and the characteristic impedance areexplained based on the results of simulations in the case of forming theground plate 22 by a plurality of ground plates, separated by slit-likespaces, with reference to FIGS. 3 to 6.

FIG. 3A is a cross-sectional view of one transmission line comprisingone signal line 21 of 1.2 μm in width and 0.6 μm in thickness, and aground plate, divided into two plates, each having a width of 50 μm, bya single slit of width w, wherein the ground plate is located spaced bya distance d from the signal line. FIG. 3B is a cross-sectional view ofa transmission line comprising the signal line shown in FIG. 3A and aground plate divided into four parts, having a width of 25 μm, by threeslits, each having a width of w/3, and these four parts are separated bythree slits each having a width of w, wherein the ground plate isseparated from the signal line by a distance of 0.6 μm.

FIG. 4 is a schematic diagram showing the capacitance between one signalline 21 and a ground plate 22 having a slit, and the other signal line23. It is assumed that the capacitance between the signal line 21 andthe ground plate 22 is represented by C_(g), and the capacitance betweenthe signal line 21 and the other signal line 23 is represented by C13.

FIG. 5 is a diagram showing the relationship between the total slitwidth and a value of C₁₃/C_(g), wherein three lines are shown, in whichthe dashed line shows the case that the ground plate is divided by asingle slit and the distance between the ground plate and the signalline is 0.6 μm, the dashed and dotted line shows the case that theground line is divided by a single slit and the distance between theground plate and the signal line is 1.8 μm, and the solid line shows thecase that the ground plate is divided into four parts and the distancebetween the ground plate and the signal line is 0.6 μm. It is noted thatthe value of C₁₃/C_(g) represents the magnitude of the coupling betweenone signal line 21 with the other signal line 23.

In FIG. 5, when an allowable value of the coupling is assumed to beC₁₃/C_(g)=0.2, the total slit widths in the respective cases must berestricted to within 3 μ/m, 4.5 μm, and 9 μm or less, and the total slitwidth cannot exceed these values.

<Third Embodiment>

FIG. 7 is a diagram showing the structure of a semiconductor integratedcircuit according to the third embodiment of the present invention.

As shown in FIG. 7, in the signal transmission line of the microstripstructure realized in a semiconductor device according to the presentembodiment, at least one hole 33 (normally, a plurality of holes 33) isformed on the signal line and the ground plate of the microstripstructure composed of the signal line 31 and the ground plate 32.

In a signal transmission line of the microstrip structure, thecharacteristic impedance Z is expressed by the equation,Z=(L/C)^(1/2)where, C represents the inter-wiring capacitance at an unit length, andL represents the inductance of the wiring.

The provision of a hole (or a plurality of holes) in both of the signalline 31 and the ground plate 32 lengthen the electric force linegenerated between the signal line 31 and the ground plate 32 and thedistance between the signal line 31 and the ground plate 32 isapparently elongated equivalently, which results in a decrease of theinter-wiring capacitance C.

When the velocity of an electro-magnetic wave is represented by v, thevelocity v is a constant and is expressed by the following equation.v=1/(LC)^(1/2)

Thus, when the capacitance C decreases, the wiring inductance Lincreases.

Consequently, it becomes possible to suppress the reduction of themaximum input capacitance of the circuit connected at the end of thesignal transmission line, and it is not necessary to increase the numberof wiring layers.

The size of the hole 33 in the ground plate 32 shown in FIG. 7 can bedetermined as follows. The size of the hole is determined to reduce theAC coupling between two adjacent signal lines 31 on the hole 33 in theground plate 32 as much as possible, and the size of the hole isdetermined to increase the characteristic impedance between the signalline 21 and the ground plate 32 as much as possible.

The number of holes is determined so as to reduce the AC couplingbetween two signal lines 31 adjacent on the hole 33 in the ground plate32 as much as possible, and so as to increase the characteristicimpedance between the signal line 31 and the ground plate 32 as much aspossible.

It is noted that it may be possible, instead of forming holes in thesignal line and the ground plate, to constitute the signal transmissionline by forming the signal line and the ground plate by a plurality ofstrips, and by connecting these strips at, for example, both terminationends of these strips so as to provide a plurality of slit-like holesbetween each of these strips.

The present invention was described above in detail by explaining thefirst to the third embodiments. However, the present invention is notlimited to these three embodiments described above, but variants thereofcan be envisaged without exceeding the scope of the present invention.

For example, the present invention includes the case of both of thesignal line and the ground plate having holes together with slits.

Furthermore, in the above embodiments, one signal line is disposed onthe ground plates. However, the ground plate may be disposed on thesignal line, and the ground plate may occupy the most significantposition. Such a disposition allows the ground plate to exhibit ashielding effect. In this case, the ground plate may be formedsimultaneously with the formation of the bonding pads on the passivationfilm, which simplifies the manufacturing process.

As described above, in a signal transmission line of a microstripstructure composed of the signal line and the ground plate, the presentinvention shows a notable effect in that the capacitance between wiringcan be decreased and the characteristic impedance between the signalline and the ground plate can be increased by forming holes in thesignal line or in the ground plate.

The other effect of the present invention is that the coupling betweenone signal line with another signal line through the ground plate can bereduced by forming holes in the ground plate.

1. A semiconductor integrated circuit comprising a microstrip structurecomprising: a signal line; a ground plate; and another signal linedisposed on an opposite side of said ground plate as said signal line;wherein, a plurality of slit holes are formed by forming said signalline of a plurality of thin strips and by connecting the thin stripsboth at respective terminal ends of the thin strips in a same layer assaid signal line and using a same material as said signal line and at aportion of the thin strips other than the respective terminal ends ofthe thin strips in the same layer as said signal line and using the samematerial as said signal line, the plurality of slit holes being formedbetween each of the strips, and an inner wall of said plurality of slitholes is only directly electrically connected to said signal line,wherein a width of each of the slit holes is smaller than a width of thesignal line.
 2. The semiconductor integrated circuit according to claim1, wherein a width of each of the slit holes is smaller than a width ofeach of the respective plurality of thin strips.